Implement Webhook interface

[dariowho]

AS A Chatbot user
I WANT Agents to run Intent fulfillment procedures
SO THAT I can use Agents that actually do something (e.g. book a hotel room), and support more sophisticated conversation flows

Details

Webhook fulfillment calls are at the core of many prediction services. The proposal is to equip Intent subclasses with an optional fulfill() method (signature to be defined).

• When fulfill(...) is defined on an Intent class, DialogflowEsConnector will enable fulfillment on that intent in the exported Agent.
• If there is at least one Intent with fulfillment enabled, and DialogflowEsConnector is instantiated without webhook information, a warning should be issued
• The Connector interface should include abstract methods to parse fulfillment requests and provide responses accordingly
• DialogflowEsConnector should implement fulfillment-related methods
• A new Webhook interface should specify how webhooks are called and used
• FlaskWebhook should provide a baseline implementation of the Webhook interface. It shoud take a Connector instance as a parameter, and wrap fulfillment-related methods around a simple Flask service